Two-Dimensional Electron-Positron Momentum Measurement at a Copper Single-Crystal Surface

Abstract
We report the first two-dimensional momentum measurements of annihilating positron-electron pairs at a solid surface. Positrons of 18 keV or 740 eV impinged on a clean Cu(121) surface. The 740-eV spectrum was resolved into two components, one associated with energetic positronium emission displaced from zero momentum and a second centered on zero momentum with distinct asymmetry for the directions parallel and perpendicular to the surface. The 18-keV spectrum was dominated by positron Bloch-state annihilation.